Excerpts from Claus Reinke's message of Wed Jun 30 03:36:00 -0400 2010:
Every Monad should try to do its best with fail. If, at some point, fail in Monad gets replaced with throwError in MonadError, or with mzero in MonadZero, or with something else, we can ignore fail in Monad, but I would still expect pattern match error via fail to integrate well with at least one of MonadPlus or MonadError.
I respectfully disagree. It is far too easy to use fail in a stack of monad transformers and end up using the wrong instance and getting error "message" semantics; it is far safer to use the appropriate MonadPlus function. fail is a disaster, from both an engineering and theoretical standpoint, and I cannot in good conscious recommend that people use it. Cheers, Edward